making WP the default load page

Hi,
Hope someone can help… I have installed WP on a /blog directory on the server. Someone advise that to make this blog your default page, you can either reinstall the blog onto the root directory or you can change the settings in the index.php (just change line 4 to point to the /blog directory instead of root) file and then move this file to root. I have tried this method but I am getting an error message. Hope someone can tell me why I am getting this error message and how to rectify the problems… The message is below :

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Warning: main(./wp-blog-header.php): failed to open stream: No such file or directory in /home/singa01/public_html/index.php on line 4

Warning: main(./wp-blog-header.php): failed to open stream: No such file or directory in /home/singa01/public_html/index.php on line 4

Fatal error: main(): Failed opening required ‘./wp-blog-header.php’ (include_path=’.:/usr/lib/php:/usr/local/lib/php’) in /home/singa01/public_html/index.php on line 4
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Thanks in advance…

Steven